1. Field of the Invention
The present invention relates to a sheet material decurling apparatus for use in machines having a sheet material conveyor, and particularly for use in printing presses.
Paper and metal foil sheet materials behave in both an elastic and a plastic manner. If deformed slightly, for example, by stretching or bending, they will return to their original shape and position when the deforming force is removed. Thus, the sheet materials exhibit elastic characteristics. However, if deformed to a much larger degree, for example, by folding, they will not return to their original shape and position when the deforming force is removed. Thus, the sheet materials exhibit plastic characteristics.
When such sheet materials are deformed beyond their elastic limit, that is when plastically deformed, for example in sheet handling machinery, they acquire a curl which makes them difficult to subsequently manipulate and stack.
Curl results from the tendency of the sheets to adhere to the inked, tacky blanket cylinder surface in certain printing presses beyond the plane tangent to both the blanket and impression cylinders of those presses. This tendency causes the sheets to be pulled from the blanket cylinder through a small radius and, thus, to be plastically deformed.
Sheet material, primarily paper, can also acquire curl from other causes such as non-uniform moisture distribution during the forming process and change in the sheet structure during the printing or finishing operation.
2. Description of the Prior Art
Various apparatus for removing a primary curl from sheet material are presently known. For example, U.S. Pat. No. 3,076,492 (Monks) discloses a sheet decurler which includes an open reverse-bending suction trough that extends transversely of the sheet. The sheet receives a reverse curvature when it is drawn into the trough by suction.
Another known, commercially available sheet decurling apparatus includes an extruded aluminum decurler bar having a trough-like shape. A slot is disposed at the vertex of the trough and is connected to a vacuum generating unit.
Prior art devices of the type generally described above have certain drawbacks, however. They are not capable of effectively decurling sheet materials which have different dimensions without certain modifications. For example, exposed portions of a vacuum conduit or slot are usually covered with adhesive tape when less than full width sheets are decurled. Further, most efficient use of the vacuum source is not achieved.